1. Field of the Invention
The present invention relates generally to tape automated bonding (TAB) of thin film structures and, more particularly, to a bumpless, single point TAB process and structure.
2. Description of the Related Art
In the manufacture of electronic devices (for example, thin-film structures such as for thermal ink-jet resistor array devices or semiconductor integrated circuits (IC), it has become feasible to make a large number of metallic interconnections to microscopic pads on the structure by utilizing a roll-type, metal film cut to form an individual set of leads, commonly known as "beam tape." The beam tape can be patterned in customized sections to match the interconnections required for the particular application. At an automated bonding station, the beam tape and structure are brought together by various known processes to connect the leads to the structure.
Generally, a precision ground thermode applies heat and pressure to the metal beams and underlying, raised, interconnection weld-spot bumps (known as "bonding pads") which lie generally atop the structure to be connected. Both high temperature thermocompression welding and lower temperature, lower pressure solder reflow welding are known, particularly for "gang-bonding" (where all welds are made simultaneously on the structure). Individual circuits may be bonded to inner leads of the beam tape and then tested while still in continuous tape form.
Two-layer beam tape is fabricated with a polyimide film to facilitate the bonding process where it is desirable to have such a dielectric barrier between the lead traces and the structure to be interconnected.
A known, typical technique is described in U.S. Pat. No. 4,551,912 (Marks, et al.). In that patent, a method of forming semiconductor chips is shown wherein a continuous sheet TAB material is moved to a cutting station. By computer control, a laser forms a "footprint" (pattern for the device to be attached), the material is moved to a bonding station where it is aligned with the chip, and a computer controlled bonding tool (an ultrasonic probe) sequentially bonds each lead in the pattern to a corresponding bump (raised interconnection weld spot) previously fabricated into the chip structure.
Typical bump height is 1.0 to 1.5 mils above the die major surface. Alternatively, prebumped beams are provided by beam tape manufacturers.
A difficulty with the Marks, et al. system is the requirement of the bumps. Column 4, lines 4-27. Basically, this technique is used to prevent short circuits from occurring between the lead and the peripheral edge of the semiconductor material of the substrate. In addition to IC fabrication process difficulties and expenses added to the chips to include the bonding bumps, in other technologies, such as a thin-film structure for an ink-jet printhead, topology requirements may severely limit or even prohibit the inclusion of bumps.
Moreover, heat bonding, such as by laser or thermode probes, may be inapplicable to structures not designed to withstand high temperature excursions.
Hence, there is a need for a heatless and bumpless TAB technique.